It is known in the prior art to use hydraulic impacts due to electrical discharge to disintegrate calculi in the urinary tract as is shown in U.S. Pat. No. 3,902,499. The prior art as taught in U.S. Pat. No. 3,902,499 employes solid state switches known generally as thyristors in the spark discharge circuits. It is commonly known that thyristors have disadvantages. Only limited values of voltage can be applied from anode to cathode in the device "off state" without destruction of the device. In order to obtain sufficient voltage to discharge the lithotrite electrodes with sufficient energy to fragment bladder calculi the thyristors have to be connected in a series arrangement of three or more devices so that the voltage across each thyristor is within its rating. With series arrangements of thyristors a critical timing requirement exists on the gate trigger signals in order to provide simultaneous turn on of all the thyristors. Application of gate signals with wave fronts less than 1 microsecond apart are required. If all the thyristors do not turn on simultaneously one or more of the devices will experience an anode to cathode voltage beyond its maximum rating causing destruction of the device or devices, as the case may be. Even with the gate signal requirement met, variations in the turn-on characteristics between individual thyristors preclude simultaneous turn-on of all thyristors in a series arrangement thereby seriously reducing the reliability of such a circuit configuration or its practicability.
Another disadvantage of the use of thyristors in such equipment is that the cost is high for each device that meets the high voltage, high current requirements of the instrument described. Use of more than one thyristor in a series arrangement further increases the instrument cost.
A further disadvantage in the use of thyristors pertains to the use of a pulse generator as taught in the prior art U.S. Pat. No. 3,902,499 having means of varying the pulse duration of the pulses applied to the thyristor gates thereby controlling the interval the capacitors will be discharged through the thyristors when the thyristors are in the "on state". Varying pulse width is a disadvantage in that the "on state" interval of thyristor is not controllable by the duration of the gate signal. In these thyristors the initiation of the "on state" does indeed occur with application of the gate signal but termination of the "on state" requires the removal of the anode voltage to a level below that value necessary to sustain the minimum holding current of the device.